Many kinds of electric light adjusting devices which reflect light according to the level of a voltage applied from the outside have been proposed (for example, JP-A-9-305092). Among them, a liquid crystal light adjusting device has been drawing attention since the structure or the control method is simple.
The liquid crystal light adjusting device is mainly formed by a polarizing plate and guest-host (GH) cells including liquid crystal molecules and dichroic dye molecules (coloring matter). Light transmitted through the polarizing plate is incident on the guest-host cells. In this case, the amount of light transmission at the emission side is relatively large (light transmittance is high) if the angle between the long axis direction of a liquid crystal molecule and the light traveling direction is small, and the amount of light transmission at the emission side is relatively small (light transmittance is low) if the angle between the long axis direction of a liquid crystal molecule and the light traveling direction is close to the right angle.
In addition, liquid crystal molecules are largely divided into two types of positive and negative types. In the case of a negative type liquid crystal molecule, the optical axis direction and the long axis direction of the liquid crystal molecule become parallel when no voltage is applied, and the optical axis direction and the long axis direction of the liquid crystal molecule become perpendicular to each other when a voltage is applied. Dichromatic dye molecules are arrayed in the same direction as liquid crystal molecules. Accordingly, when the negative type liquid crystal molecule is used as a host, the light transmittance is relatively high (bright) when no voltage is applied, and the light transmittance is relatively low (dark) when a voltage is applied.
On the other hand, in the case of a positive type liquid crystal molecule, the arrangement of liquid crystal molecules when no voltage is applied and the arrangement of liquid crystal molecules when a voltage is applied are opposite those in the case of a negative type liquid crystal molecule. Accordingly, when the positive type liquid crystal molecule is used as a host, the light transmittance is relatively low (dark) when no voltage is applied, and the light transmittance is relatively high (bright) when a voltage is applied.